a. Field of the Invention
This invention pertains to omnidirectional antennas. More particularly, this invention pertains to antennas that provide hemispherical omnidirectional coverage or coverage of selected sectors of a hemisphere for use in cellular communication systems.
b. Description of the Prior Art
A simple quarter-wave length vertical conductor mounted on, and feed in opposition to, a ground plane provides an omnidirectional radiation pattern in azimuth. Classical antennas of this type are well known in the art. Such an antenna, however, has a null in the radiation pattern at the zenith, i.e. directly above the vertical conductor. In many applications, the null at the zenith is not important. However, the relatively recent development of cellular communications systems has brought with it a requirement for an omnidirectional pattern with no null at the zenith and in some circumstances for coverage of selected sectors of a hemisphere. For instance, an objective of a cellular communication system may be to provide coverage throughout one room from one antenna mounted on the ceiling of the room or to provide coverage throughout a building from one antenna mounted under the roof of the building. In such instances, in order to provide communications coverage throughout the hemisphere below the antenna, the antenna must not only provide omnidirctional coverage in azimuth, but must also not have a null in the radiation pattern immediately below the antenna.